The mechanism of transport of thyroid hormones into skeletal muscle cells was studied in two systems, the intact rat soleus muscle and continuously cultured rat myoblasts (L6E9). In intact muscle, uptake of L-T3 was enhanced by insulin, which appeared to act through the Na+/H+ exchanger system. In myoblasts, transport of L-T3 to cell nuclei was investigated and shown, by comparison with D-T3, to be regulated by a stereospecific mechanism probably located at the plasma membrane. It was also shown, by the use of specific inhibitors, that the transfer of T3 from plasma to the cell nucleus is temperature dependent, requires energy and an intact cytoskeletal architecture, and is probably mediated through an endocytotic pathway. These results indicate further that thyroid hormone uptake into cells may be under metabolic control.